Cooled gas turbine guide blade for a gas turbine, use of a gas turbine guide blade and method for operating a gas turbine

ABSTRACT

Described is a cooled gas turbine guide blade for a gas turbine, with a hollow blade profile which comprises an onflow edge onto which a working medium is capable of flowing, and with, for guiding a cooling medium, an onflow edge duct running inside the blade profile along the onflow edge. Furthermore, to the use of such a gas turbine guide blade and to a method for operating a gas turbine with an abovementioned gas turbine guide blade. In order to provide a gas turbine guide blade with an increased lifetime by means of the invention, it is proposed that, in the onflow edge duct, an electrical heating element be provided, which extends approximately completely over the entire length of the onflow edge duct and through which a heating current flows, before the operation of the gas turbine, for the preheating of the gas turbine guide blade.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional application of U.S. patent application Ser. No.11/479,183, filed Jun. 30, 2006, which claims the benefits of EuropeanPatent application No. 05014377.5 filed Jul. 1, 2005. Both applicationsare incorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The invention relates to a cooled gas turbine guide blade for a gasturbine, with a hollow blade leaf which comprises an onflow edge ontowhich a working medium is capable of flowing, and which has, for guidinga cooling medium, an onflow edge duct running inside the blade profilealong the onflow edge. The invention relates, furthermore, to the use ofsuch a gas turbine guide blade and to a method for operating a gasturbine having an abovementioned gas turbine guide blade.

BACKGROUND OF THE INVENTION

A cooled turbine guide blade for a gas turbine is known from EP 1 505256 A2. The turbine guide blade has a leading edge of a blade profile, ahot gas being capable of flowing onto said leading edge. A cooling ductrunning transversely with respect to the hot gas direction extendsinside.

The calculation of the maximum permissible lifetime of gas turbineblades of this type, so as to ensure a fault-free and reliable operationof the gas turbine, is also carried out, taking into account the fatiguelifetime in terms of fatigue strength under short-term vibratorystresses (low-cycle fatigue=LCF). The LCF lifetime is determined on thebasis of theoretical models and on the assumption of boundaryconditions. After the maximum permissible lifetime is reached, the gasturbine is opened and the gas turbine blades are investigated visuallyand mechanically for defects, such as, for example, cracks. Theinvestigation is intended to ascertain whether its further use for anext operating interval is possible, without the operation of the gasturbine being put at risk.

The dismantling of the gas turbine is complicated and leads to anundesirable increase in downtime. Furthermore, the investigations aretime-consuming and cost-intensive, and therefore there is a generaleffort to reduce the downtimes of the gas turbine and increase thelifetime of the components used and employed.

SUMMARY OF THE INVENTION

The object of the invention is, therefore, to provide a gas turbineguide blade with an increased lifetime. A further object of theinvention is the use of such a gas turbine guide blade and thespecification of two methods for operating a gas turbine having a gasturbine guide blade, in order to increase its lifetime.

The object aimed at the gas turbine guide blade is achieved by means ofa gas turbine guide blade according to the claims. The inventionproposes that the generic gas turbine guide blade has in the onflow edgeduct an electrical heating element which extends approximatelycompletely over the entire length of the onflow edge duct.

The invention is based on the recognition that particularly sharplyrising or sharply falling temperatures, even what are known astemperature shocks, may give rise to cracks in the blade material andpromote, if not even accelerate, the growth of cracks. The rapidtemperature changes cause in each case thermal stresses in the bladematerial which has to withstand many such temperature alternations andstress alternations. On account of an excessively large number oftemperature alternations, in particular cold starts of the gas turbine,the blade material suffers fatigue. Under the progressive action oftemperature alternations and stress alternations, cracks may arise andgrow further. If the cracks overshoot a critical length or if too largea number of cracks of uncritical length are present within a unit ofarea, then the component has to be exchanged.

These negative indications occur more frequently, the more frequentlythe blade material acted upon by hot gas is exposed to rapid temperaturechanges, and the greater the temperature differences to be withstood ineach case are.

Particularly during the cold start of the gas turbine and while the gasturbine is being run down, the highest temperature differences occur inthe components acted upon by hot gas.

However, the unplanned shutdown of the gas turbine under full load inthe event of a critical fault has the greatest adverse influence on thelifetime of the turbine blade. This situation, designated as a trip,significantly reduces the lifetime of the components acted upon by hotgas and therefore also that of gas turbine blades, since their materialhas at these moments to withstand particularly high temperaturegradients and/or temperature fluctuations.

This is where the invention comes in. In a preferably stationary gasturbine, the lifetime of the gas turbine guide blade is significantlyincreased in that, to achieve the object aimed at the method, accordingto the claims, the gas turbine guide blade is preheated before the coldstart of the gas turbine. For this purpose, the gas turbine blade has inthe onflow edge duct a heating element which extends completely over thelength of the blade profile. Even before the firing of the gas turbine,that is to say during preheating, the electrical heating element throughwhich an electrical current flows warms up comparatively slowly the gasturbine guide blade which at the beginning is at room temperature, sothat the temperature-induced thermal stresses in the blade material canbe kept correspondingly low. The temperature rise takes placecomparatively slowly, not quickly or abruptly, as in the previousstarting, that is to say firing, of the gas turbine. With the firing ofthe gas turbine, the heating of the gas turbine guide blade isestablished.

Likewise, to achieve the object aimed at the method, according to theclaims, the lifetime of the gas turbine guide blade can be increased ifthe reheating of the gas turbine guide blade is commenced when thefiring of the gas turbine is shut down, planned or unplanned in theevent of a full-load shutdown (trip). Since the gas turbine guide bladeis heated to its maximum permissible operating temperature during theoperation of the gas turbine, a directed and controlled lowering of thetemperature of the blade material can be achieved by means of thereheating. As a result, the temperature lowering duration is appreciablyprolonged, as compared with the previous normal cooling of the gasturbine guide blade, so that the material stresses occurring due to thetemperature lowering are comparatively minor.

Since a reduction in thermal stresses can be achieved in both methods,the occurrence of cracks in the blade material can be further delayedand the growth of cracks slowed, as compared with previous gas turbineblades from the prior art. The methods which thus take care of thematerial lead to a significant rise in fatigue strength under short-timevibratory stresses and to a prolongation of the gas turbine guide bladelifetime guaranteed to be uncritical in terms of faults. A gas turbineequipped with the gas turbine blades and/or operated according to themethods can be employed reliably and in a fault-free way for longer,thus having a cost-diminishing effect because checking investigationsare shifted further back in time.

Since the gas turbine blades of the first and the second turbine stagehave to withstand the highest operating temperatures, the greatesttemperature differences also occur in these gas turbine blades. Theoperating temperatures lie in such high temperature ranges that thesegas turbine blades, in particular, are cooled. The invention istherefore particularly suitable for cooled gas turbine blades. Bycontrast, uncooled gas turbine blades are subject to lower temperaturedifferences, so that the described influence of the temperature changesbetween hot (=operating temperature of the component) and cold (=ambienttemperature) is less.

Advantageous embodiments are specified in the subclaims.

A particularly uniform warming of the blade leaf of the gas turbineguide blade can be achieved if the electrical heating element bearsagainst the duct wall of the onflow edge duct or is at least partiallyintegrated in the duct wall. This particularly beneficial thermalcoupling of heating element and duct wall leads to a low-loss heating ofthe material of the gas turbine guide blade.

In order to ensure a particularly uniformed distribution of the heatenergy radiated by the heating element into the blade material, theheating element is designed as a helical heating coil. The heating coilis known and is available cost-effectively.

In order to achieve a sufficient warming of the gas turbine guide bladebefore the starting of the gas turbine, the thermal output supplied isincreased during the heating operation. This leads, during thepreheating, to a permanent and continuous warming of the blade materialup to a temperature value, of which the difference from the operatingtemperature of the gas turbine guide blade is low, as compared with thedifference between the ambient temperature and operating temperature. Asa result, material stresses occurring in the blade material build upslowly and therefore carefully in terms of the material. When firingsubsequently commences with the starting of the gas turbine, theremaining temperature rise is low. Temperature shocks are thus avoided.It is also possible, by means of the electrical heating element, tomaintain the gas turbine guide blade at a “warm” standby temperaturelying near operating temperature, in order to achieve a shortenedstarting phase.

During the reheating phase, the thermal output supplied to the gasturbine guide blade is lowered continuously, in order to bring about acomparatively slow lowering of the temperature of the blade material,thus leading to reduced thermal material expansions and stresses.

Overall, by means of the invention, the LCF fatigue of gas turbine guideblades can be reduced, thus having a positive effect on their lifetimeand bringing about a reduction in the operating risk of a gas turbineequipped with such gas turbine blades.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained with reference to figures in which:

FIG. 1 shows a gas turbine in a part longitudinal section,

FIG. 2 shows a gas turbine guide blade with a helical heating coil, and

FIG. 3 shows in cross section a gas turbine guide blade according to theinvention from FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a gas turbine 1 in a part longitudinal section. It has,inside, a rotor 3 which is rotary-mounted about an axis of rotation 2and which is also designated as a turbine rotor. An intake casing 4, acompressor 5, a toroidal annular combustion chamber 6 with a pluralityof burners 7 arranged rotationally symmetrically with respect to oneanother, a turbine unit 8 and an exhaust casing 9 succeed one anotheralong the rotor 3. The annular combustion chamber 6 forms a combustionspace 17 which communicates with an annular hot-gas duct 18. There, fourturbine stages 10 connected in series form the turbine unit 8. Eachturbine stage 10 is formed from two blade rings. As seen in the flowdirection of a hot gas 11 generated in the annular combustion chamber 6,a guide blade row 13 is followed in the hot-gas duct 18 in each case bya row 14 formed from moving blades 15. The guide blades 12 are fastenedto the stator, whereas the moving blades 15 of a row 14 are attached tothe rotor 3 by means of a turbine disk. A generator or a working machine(not illustrated) is coupled to the rotor 3.

A gas turbine guide blade 30 according to the invention is illustratedin longitudinal section in FIG. 2. The gas turbine blade 30 has a bladeprofile 32 which extends between two platforms 34. The platforms 34forms the radially outer and inner boundary of the hot-gas duct 18 inwhich the hot-gas 11 flows when the gas turbine is in operation. Theonflow-side leading edge of the blade profile 32 is designated as theonflow edge 36. At the opposite end, the blade profile 32 has a bladetrailing edge 37 at which the hot gas 11 flows off.

The blade profile 32 may have a plurality of cavities 38, into which acooling medium 40, preferably cooling air, supplied by means of theplatform 34 can flow and cool the cast turbine blade 30. For cooling,the customary cooling methods, such as convection cooling, bafflecooling and/or film cooling and also effusion cooling, may be employed.

In order to preheat the gas turbine guide blade 30 before the cold startof the gas turbine 1, an electrical heating element 50 is provided inone of the cavities 38, but preferably in the cavity which follows theonflow edge 36, that is to say in the onflow edge duct 40. The heatingelement 50 extends approximately over the complete length of the onflowedge 36, that is to say from the radially inner platform 34 a to theouter platform 34 b.

In order to allow a particularly uniform warming of the gas turbineblade 30, the heating element 50 is designed as a helical heating coil54.

FIG. 3 shows a cross section through the gas turbine blade 30 accordingto the invention, as shown in FIG. 2. The onflow edge duct 42 hasarranged in it the helical heating coil 54, through which a regulatableelectrical heating current can flow in order to preheat or reheat thegas turbine guide blade 30 so as to prolong the lifetime of the latter.The electrical heating current gives rise in the heating coil 54 to heatradiation which, before the operation of the gas turbine 1, warmscomparatively slowly the material of the cast gas turbine guide blade30, preferably the material of the blade profile 32 and of the platform34, in order to cause the thermomechanical material stresses to grow inan order of magnitude which is uncritical for the LCF lifetime or tocause them to fade away after operation.

By virtue of the preheating or reheating of the gas turbine guide blade30, particularly sharp temperature fluctuations, what are known astemperature shocks, arising in a comparatively short time interval canbe avoided, thus leading to an increase in the fatigue strength undershort-time vibratory stresses and effectively reducing the occurrence ofcracks or the growth of cracks in the blade material.

1. A method for operating a gas turbine engine, comprising: preheating aguide blade of the gas turbine engine by providing a heat input to theguide blade before a start-up operation of the gas turbine engine;initiating the start-up operation of the gas turbine engine; adjustingthe heat input to the guide blade during a normal operating state of thegas turbine engine; and adjusting the heat input to the guide bladeduring a shutdown operation of the gas turbine engine.
 2. The method asclaimed in claim 1, wherein the heat input to the guide blade is variedduring the start-up operation.
 3. (The method as claimed in claim 2,wherein the heat input to the guide blade is increased during thestart-up operation
 4. The method as claimed in claim 3, wherein the heatinput to the guide blade is constant during the start-up operation. 5.The method as claimed in claim 1, wherein the heat input to the guideblade is decreased during the normal operation.
 6. The method as claimedin claim 5, wherein the heat input to the guide blade is constant duringthe normal operation.
 7. The method as claimed in claim 1, wherein theguide blade is reheated after the shutdown operation of the gas turbineengine.